Table 1 summarizes the expression profile of the TetO-H2B-GFP reporter transactivated by the MMTV-tTA(NIH) transgene. Based on these observation using two different TetO-driven reporter transgenes, it is evident that the original MMTV-tTA(NIH) transgenic strain has limited applicability for the targeted expression of transgenes in the mammary epithelium in the FVB genetic background. In particular the activation of the MMTV-tTA in the skin makes it difficult to use this strain for in vivo imaging of the normal mammary gland and to monitor the development of mammary tumors. The gene discussed is H2BC21; the disease is breast cancer.